1. Field of the Invention
This invention relates generally to apparatus for transmitting an AM stereophonic signal and, more particularly, is directed to apparatus for transmitting an AM stereophonic signal of the independent side band type.
2. Description of the Prior Art
Systems for transmitting AM stereo signals are known in the art. In one known system, a matrix circuit produces a sum signal (L+R) corresponding to the sum of left (L) and right (R) channel stereophonic signals and a difference signal (L-R) corresponding to the difference of the left (L) and right (R) channel stereophonic signals. The sum and difference signals are thereafter phase shifted to provide a phase difference of 90.degree. between the two signals. Thereafter, the phase shifted sum signal is multiplied with a carrier signal cos .omega..sub.c t and the phase shifted difference signal is multiplied by a carrier signal -sin .omega..sub.c t. The two multiplied signals are then combined with each other, along with the carrier signal cos .omega..sub.c t, and the resultant signal is transmitted through a transmitting antenna as an independent side band (ISB) AM stereophonic signal.
With the above system, because of the phase-shifting arrangement, the side band wave .omega..sub.c -P of the left (L) channel signal is produced at a lower side band (LSB) of the carrier .omega..sub.c. In like manner, the side band wave of the right (R) channel signal is generated at the upper side band (USB) of the carrier .omega..sub.c. However, with such systems, if the signal is biased to either side of the left (L) channel or the right (R) channel, the resultant signal becomes a single side band (SSB) wave which generally contains a second order distortion component in the envelope thereof. Accordingly, with an AM stereophonic receiver which uses a conventional diode envelope detector, a relatively large distortion is produced, for example, a maximum distortion of the order of approximately 13%.